In this article I wanna quickly explain what the difference between these items are and why currently it's best to choose the default one.
1. Power Up(100%)
It means that all your earnings from the post will go to power your account up. In order to withdraw, you need to power down which will take 13 weeks with installments paid in Steem every week. One benefit for you of Powering up 100% is that it increases your voting power, which is good to have.
2. Default(50%/50%)
This option will split the rewards and will use half of it to power your account up and will transfer the other half into your account in Steem dollars or SBD which is pegged to the US dollar. You can convert your SBDs into Steem through either the internal market on steemit.com or through an external market.
3. Decline Payout
As the option clearly states it, you won't be rewarded. Why would anyone choose this option? I honestly don't know! But there are people who choose this option for at least some of their posts. I haven't yet tried to figure out why they do so!